Absent from the original, it is learned by Psaro at level 42 in the remakes.
Kathwack is learned by mastering the Sage vocation, and costs 15MP.
Angelo can acquire the spell by investing 56 points into his Staff skill and costs 15MP.
Kathwack is learned by priests at level 55, and costs 20 MP to cast. As with the other whack spells, it's potency is reliant on the priest's Magical mending, raising the base accuracy from 75% at 200 to 100% at 799.
Kathwack appears as Kiryl's Coup-de-Grâce, carrying over from Dragon Quest: Monster Battle Road. After trying and failing to cast Thwack multiple times, Kiryl becomes annoyed and stomps on the text windows in frustration, before casting a souped-up version of Kathwack, which annihilates groups of enemies with waves of darkness. This version of Kathwack causes damage, rather than instant death. This is a reference to Kiryl's AI in the NES version of Dragon Quest IV, where Kiryl would infamously cast the Whack and Thwack spells constantly in battle to no effect.